Ventilation fans are common to electronic devices, such as personal computers, photocopy machines, video cassette recorders, which generate heat during operation and require such heat to be removed to maintain an adequate working temperature. Conventionally, such ventilation fans are brushless direct current driving fans which comprise a central coil or winding, around which pole plates are disposed to be magnetized when electricity is supplied to the central coil through a circuit board. A fan blade assembly having a permanent magnet mounted thereon is rotatably supported on the central coil with the permanent magnet disposed around the pole plates in the close proximity thereof. The circuit board has a magnetic polarity sensing device for sensing the magnetic polarity of the permanent magnet of the blade assembly passing through a particular position and with the sensing result, the circuit board changes the magnetic polarities of the pole plates so as to have the pole plates maintained at suitable magnetic polarities for expelling the blade assembly to rotate about the central coil.
The conventional DC fan has several disadvantages. For example, once the DC fan is forcibly stopped by an external resistance which is larger than the expulsion generated between the pole plates and the permanent magnet, the DC fan will not resume rotation upon removal of the external resistance for the DC fan will stop at the location where the magnetic polarities of the permanent magnet oppose the pole plates having opposite magnetic polarities and is attracted thereby. Due to the uniform magnetic force distribution on the rotator of the DC fan, the DC fan will be in a force equilibrium state even after the external force is removed. This prevents the blades of the fan from rotating again once the resistance is removed. Under this situation, the fan has to be re-started or manually moved to overcome such attraction force between the fan permanent magnet and the pole plates and thus resume rotation.
Furthermore, in the conventional design, there is no lubrication sealing device so that the lubricant film which exists between the hub hole of the blade assembly and the axle on which the blade assembly is mounted and about which the blade assembly rotates may gradually leak out of the blade assembly and thus contaminate the DC fan.